1. Field of the Invention
This invention relates to a wastewater treatment system that uses simultaneous separation of solids and phosphorus sludge from agricultural and municipal wastewater and industrial effluents and to methods for the simultaneous removal of manure solids and phosphorus from municipal and agricultural wastewater.
2. Description of the Related Art
Municipal and agricultural waste disposal is a major problem. For agricultural animals, the animals are confined in high densities and lack functional and sustainable treatment systems. The liquid wastes are generally treated in large anaerobic lagoons with intermittent disposal through land applications (Stith, P. and Warrick, J., Boss Hog: North Carolina's pork revolution, The News & Observer, 1-3, Feb. 19-26, 1995; USEPA, Proposed regulations to address water pollution from concentrated animal feeding operations, EPA 833-F-00-016, January 2001, Office of Water, Washington, D.C. 20460). This system was developed in the early and mid 20th century prior to the current trend in high concentrated livestock operations. One of the main problems in sustainability is the imbalance of nitrogen (N) and phosphorus (P) applied to land (USEPA, supra; Cochran et al., Dollars and Sense: An economic analysis of alternative hog waste management technologies, Environmental Defense, Washington, D.C., 2000). Nutrients in manure are not present in the same proportion needed by crops, and when manure is applied based on a crop's nitrogen requirement, excessive phosphorus is applied resulting in phosphorus accumulation in soil, phosphorus runoff, and eutrophication of surface waters (Heathwaite et al., A conceptual approach for integrating phosphorus and nitrogen management at watershed scales, J. Environ. Qual., Volume 29, 158-166, 2000; Sharpley et al., Practical and innovative measures for the control of agricultural phosphorus losses to water: An overview, J. Environ. Qual., Volume 29, 1-9, 2000; Edwards and Daniel, Environmental Impacts of On-Farm Poultry Waste Disposal—A Review, Bioresource Technology, Volume 41, 9-33, 1992).
The change from small individual animal production operations to large, confined, commercial enterprises has caused many problems for the animal production industry including emission of ammonia (NH3) from lagoons. It may be anticipated that about 50-80% of the nitrogen (N) entering animal lagoons will escape to the atmosphere through NH3 volatilization (Miner and Hazen, Transportation and application of organic wastes to land, In: Soils for Management of Organic Wastes and Waste Waters, 379-425, eds: L. F. Elliot and F. J. Stevenson, Madison, Wis.: ASA/CSSA/SSSA; Barrington and Moreno, Swine Manure Nitrogen Conservation Using Sphagnum Moss, J. Environ. Quality, Volume 24, 603-607, 1995; Braum et al., Nitrogen Losses from a Liquid Dairy Manure Management System, I: Agron. Abstracts, Madison, Wis., ASA, 1997). Biological removal of nitrogen through the process of nitrification and denitrification is regarded as the most efficient and economically feasible method available for removal of nitrogen from wastewaters (Tchobanoglous, G. and F. L. Burton, Wastewater Engineering Treatment, Disposal, and Reuse, Boston, Mass.: Irwin/McGraw-Hill, 1991). The effectiveness of the biological nitrogen removal process depends on the ability of nitrifying organisms to oxidize ammonium ions (NH4+) to nitrite (NO2−) and nitrate (NO3−). Subsequent reduction of molecular nitrogen, denitrification may be essential as well if one desires to reduce total nitrogen as well as ammonia nitrogen. This step is rapid with available carbonaceous substrate and an anaerobic environment, conditions which are typically found in farm settings in constructed wetlands or liquid manure storage units. The reaction rate of nitrification is extremely low compared to that of denitrification, so that nitrification normally will be a rate limiting step in the biological nitrogen removal process (Vanotti and Hunt, Transactions of the ASAE, Volume 43 (2), 405-413, 2000). Nitrification of wastewater can be performed with many processes such as for example suspended-growth nitrification, attached-growth nitrification, etc. Two bacterial genera are responsible for nitrification. Nitrosomonas oxidizes ammonia to the intermediate product nitrite and nitrite is converted to nitrate by Nitrobacter. The term nitrifiers is a general term that applies to a consortia of Nitrosomonas and Nitrobacter well known in the art. Nitrifying bacteria are present in almost all aerobic biological treatment processes, but their numbers are limited. There are many aerobic processes that have been developed to favor nitrification (Tchobanoglous G. and F. L. Burton; supra). They can be separated into two main groups: suspended-growth and attached-growth (Grady, C. P. L., G. T. Daigger, and H. C. Lim. 1999. Biological Wastewater Treatment. 2nd ed. Marcel Dekker, New York, N.Y.). In suspended-growth nitrification, a nitrifying sludge composed of free bacteria is mixed with the wastewater liquid by the aeration or agitation of the liquid. The commonly used activated-sludge process is a suspended growth process that combines bacterial biological oxygen demand (BOD) removal and bacterial nitrification treatment (nitrogen removal) that are performed by separate bacteria. In other cases, carbon oxidation and nitrification functions are done in separate tanks. Attached-growth nitrification uses various media so that the nitrifying bacteria attach to the surface of the media, examples include trickling filters, rotating biological contactors, packed-bed reactors, overland flow, and others known in the art. Another type of attached-growth system is intermediate between suspended- and attached-growth and that is a fluidized bed biological reactor. In this type of reactor nitrifying pellets remain suspended in the fluid, i.e., fluidized by the drag forces associated with the upward flow of air and water. The nitrifying bacteria are attached to various light-weight media or entrapped in polymeric porous materials made of polyvinyl alcohol (PVA) or polyethylene glycol (PEG) and fluidized in the reactor tank. One of the advantages of using such nitrifying pellets is that the number of microorganisms in the reactor can be increased thus removing the ammonia more quickly. Whether a fluidized bed biological reactor, a six hour process, or suspended growth process, a two day process, is used, the changes in water characteristics after treatment are the same. All nitrifiers are autotrophic microorganisms that consume ammonia, oxygen, and carbon dioxide, and produce oxidized nitrogen (nitrate and nitrite) and acidity. Thus, the nitrification process removes both carbonate alkalinity and ammonia from wastewater and increase acidity (Vanotti et al, Trans. ASAE, Volume 46 (6), 1665-1674, 2003). In general, any nitrification process will work provided bacteria is adapted to operate at high ammonia concentrations. U.S. Pat. No. 6,893,567 to Vanotti et al., issued May 17, 2005, teaches that once ammonia and carbonate alkalinity concentrations in swine wastewater are substantially reduced with a nitrification pre-treatment, the subsequent addition of lime rapidly increases the pH of the liquid, thereby removing the soluble phosphates contained in the wastewater and promoting formation of phosphorus precipitate with small amounts of chemical added.
The basic problem related to nitrification in wastewaters with a high content of organic carbon is the low growth rate of the nitrifying bacteria; the generation time of these microorganisms is about 15 hours. Compared to heterotrophic microorganisms, which have generation times of 20-40 minutes, the nitrifiers compete poorly for limited oxygen and nutrients and tend to be overgrown or washed out of reactors (Figueroa and Silverstein, Water Environ. Res., Volume 64 (5), 728-733, 1992; Wijffels et al., Possibilities of vitrification with immobilized cells in wastewater treatment Model or practical systems, Wat. Sci. Tech., Volume 27 (5-6), 233-240, 1993). The nitrification of lagoon swine wastewater is an especially difficult process because of the very low numbers of Nitrosomonas and Nitrobacter usually found after anaerobic treatment (Blouin et al., Nitrification of swine waste, Canadian J. Microbiol., Volume 36, 273-278, 1990). Even when the oxygen supply is plentiful, an adaptation period is needed to reach a minimum bacteria concentration for effective nitrification. Recycling surplus activated sludge in an aerobic reactor or long hydraulic retention time (HRT) is required to retain slow growing autotrophic nitrifiers. Unfortunately, in the absence of enriched nitrifying populations, aerobic treatment of lagoons can potentially add to problems by stripping ammonia into the atmosphere, particularly if uncontrolled or excessive flow rates of air are used (Burton, A review of the strategies in the aerobic treatment of pig slurry: Purpose, theory, and method, J. Agric. Eng. Res., Volume 53, 249-272, 1992).
Managing agricultural sources of phosphorus and nitrogen at the watershed scale in order to reduce their impact on water quality requires a balanced and holistic approach (Heathwaite et al., J. Environ. Qual., Volume 29, 158-166, 2000). In the past, most emphasis has been placed on nitrogen management to ameliorate nitrate losses to ground water. While the high solubility and mobility of nitrate within agricultural systems may justify this emphasis, such bias ignores other critical elements, notably phosphorus.
Phosphorus inputs accelerate eutrophication when it runs off into fresh water and has been identified as a major cause of impaired water quality (Sharpley et al., 2000, supra). Eutrophication restricts water use for fisheries, recreation, industry, and drinking due to the increased growth of undesirable algae and aquatic weeds and resulting oxygen shortages caused by their death and decomposition. Also many drinking water supplies throughout the world experience periodic massive surface blooms of cyanobacteria. These blooms contribute to a wide range of water-related problems including summer fish kills, unpalatability of drinking water, and formation of trihalomethane during water chlorination. Consumption of cyanobacteria blooms or water-soluble neuro- and hepatoxins released when these blooms die can kill livestock and may pose a serious health hazard to humans. Recent outbreaks of the dinoflagellate Pfiesteria piscicida in near-shore waters of the eastern United States also may be influenced by nutrient enrichment. Although the direct cause of these outbreaks is unclear, the scientific consensus is that excessive nutrient loading helps create an environment rich in microbial prey and organic matter that Pfiesteria and menhaden (target fish) use as a food supply. In the long-term, decreases in nutrient loading will reduce eutrophication and will likely lower the risk of toxic outbreaks of Pfiesteria-like dinoflagellates and other harmful algal blooms. These outbreaks and awareness of eutrophication have increased the need for solutions to phosphorus run-off.
Past research efforts on phosphorus removal from wastewater using chemical precipitation have been frustrating due to the large chemical demand and limited value of by-products such as alum sludge, or because of the large chemical demand and huge losses of, ammonia at the high pH that is required to precipitate phosphorus with calcium (Ca) and magnesium (Mg) salts (Westerman and Bicudo, Tangential flow separation and chemical enhancement to recover swine manure solids and phosphorus, ASAE Paper No. 98-4114, St. Joseph, Mich.: ASAE, 1998); Loehr et al., Development and demonstration of nutrient removal from animal wastes, Environmental Protection Technology Series, Report EPA-R2-73-095, Washington, D.C.: EPA, 1973). Other methods used for phosphorus removal include flocculation and sedimentation of solids using polymer addition, ozonation, mixing, aeration, and filtration (See U.S. Pat. No. 6,193,889 to Teran et al). U.S. Pat. No. 6,153,094 to Craig et al. teaches the addition of calcium carbonate in the form of crushed limestone to form calcium phosphate mineral. The patent also teaches adsorbing phosphorus onto iron oxyhydroxides under acidic conditions.
Continuing efforts are being made to improve agricultural, animal, and municipal waste treatment methods and apparatus. U.S. Pat. No. 5,472,472 and U.S. Pat. No. 5,078,882 (Northrup) disclose a process for the transformation of animal waste wherein solids are precipitated in a solids reactor, the treated slurry is aerobically and anaerobically treated to form an active biomass. The aqueous slurry containing bioconverted phosphorus is passed into a polishing ecoreactor zone wherein at least a portion of the slurry is converted to a beneficial humus material. In operation the system requires numerous chemical feeds and a series of wetland cells comprising microorganisms, animals, and plants. See also U.S. Pat. Nos. 4,348,285 and 4,432,869 (Groeneweg et al); U.S. Pat. No. 5,627,069 to Powlen; U.S. Pat. No. 5,135,659 to Wartanessian; and U.S. Pat. No. 5,200,082 to Olsen et al (relating to pesticide residues); U.S. Pat. No. 5,470,476 to Taboga; and U.S. Pat. No. 5,545,560 to Chang.
U.S. Pat. No. 6,177,077 (Lee et al.) and U.S. Pat. No. 6,200,469 (Wallace) both relate to the removal of nitrogen and phosphorus from wastewater wherein the phosphate is removed using microorganism in aerobic tanks which absorb the phosphorus released from denitrified wastewater. See also U.S. Pat. No. 6,113,788 to Molof et al., U.S. Pat. No. 6,117,323 to Haggerty; U.S. Pat. No. 6,139,743 to Park et al.
There is concern about the introduction and spread of diseases through wastewater. For example, there is great concern about the spread of Foot and Mouth Disease in countries throughout the world. Major programs are in place at present in countries free of Foot and Mouth Disease to prevent the introduction or spread of the disease. The Irish Agriculture and Food Development Authority (Teagasc) implemented a 12-point Foot and Mouth Disease protection plan including restrictions in liquid manure spreading on fields allowing only emergency spreading when manure storage tanks are likely to overflow. If the disease is introduced, it could be spread as an aerosol during liquid manure spreading. The virus can persist in aerosol form for long periods. It is estimated that sufficient virus to initiate infection can be windborne as far as 100 km (Blood, D. C., Radostits, O. M., and Henderson, J. A., Veterinary Medicine, 6.sup.th addition, pages 733-737, 1983. Bailliere Tindall, London, U.K.). The virus is resistant to common disinfectants and the usual storage practices. But it is particularly susceptible to changes in pH away from neutral, or to heat treatment using autoclaving under pressure. Liquid swine manure normally has a pH of about 6 to 8, and the Foot and Mouth Disease virus can survive in this pH range. A shift in the pH in either direction below 5 and above 9 makes conditions for survival less favorable. Thus, infectivity of the Foot and Mouth Disease virus may be effectively destroyed by chemicals such as acids and alkalis (Callis, J., and Gregg, D., Foot-and-mouth disease in cattle, pages 437-439, 1986. In J. L. Howard (ed.), Current Veterinary Therapy 3. W. B. Saunders Company. Philadelphia, Pa.). Unfortunately, liquid swine manure contains inherent buffers, mainly carbonates and ammonia, that prevent changes in pH except when large amounts of chemicals are used. In addition to the large chemical need, addition of acid to liquid manure gives a sudden release of hydrogen sulfide and risk of gas poisoning. On the other hand, increase of pH 9 with the addition of alkali chemicals such as calcium hydroxide (lime) or sodium hydroxide is prevented by ammonia equilibrium. This means that the alkali is used to convert ammonia into gas form before effective increase of pH above 9 is achieved. Ammonia volatilization from animal facilities is an environmental problem in and of itself.
U.S. Pat. No. 6,893,567, issued May 7, 2005 (Vanotti et al), is directed to wastewater systems and processes for the removal of solids, pathogens, nitrogen, and phosphorus from municipal and agricultural wastewater which includes nitrification of wastewater and increasing the pH of the nitrified wastewater by adding a metallic-containing salt and hydroxide to precipitate phosphorus to form a useable effluent having a specified nitrogen:phosphorus ratio that is useful as a fertilizer or spray for remediation of contaminated soils. The system also reduces the presence of infectious microorganisms such as enterobacteriogenic bacteria and picarnoviruses. The precipitated phosphorus is recovered and used to form useable phosphorus products.
The polymer PAM is extensively used as a settling agent for food processing and packing, paper production, mine and municipal wastewater treatment, as a clarifier for sugar extraction and potable water treatment, and as a soil conditioner to reduce irrigation water erosion (Barvenick, Soil Science, Volume 158, 235-243, 1994). It has also been shown that cationic PAM are also used to substantially increase separation of suspended solids, organic nutrients, and carbon compounds from liquid animal manures (Vanotti and Hunt, Trans. ASAE, Volume 42 (6), 1833-1840, 1999; Chastain et al., Appl. Engr. Agric., Volume 17 (3), 343-354, 2001; Vanotti et al., Trans. ASAE, Volume 45 (6), 1959-1969, 2002; Walter and Kelley, Biores. Technol., Volume 90, 151-158, 2003; Timby et al., Appl. Engr. Agric., Volume 20 (1), 57-64 2004; Estevez Rodriguez et al., Appl. Engr. Agric., Volume 21 (4), 739-742, 2005; Vanotti et al., Proc. WEFTEC '05, 4073-4092, 2005(c)).
While various systems have been developed for treating wastewater for the removal of solids, pathogens, nitrogen, and phosphorus; there still remains a need in the art for a more effective wastewater treatment system. The present invention, different from prior art systems, provides a system which generates one solids stream instead of two that facilitates management and operation. It eliminates a dewatering step, and reduces the use of polymers. In the present invention, two or more sludges with contrasting chemical properties can be combined using a single application of polymer flocculants and one common dewatering equipment, which together simplifies the overall capital investment and operational costs for dewatering, an important step in wastewater treatment.